Deep Ductile Shear Zone Facilitates Near‐Orthogonal Strike‐Slip Faulting in a Thin Brittle Lithosphere

Abstract : Some active fault systems comprise near-orthogonal conjugate strike-slip faults, as highlighted by the 2019 Ridgecrest and the 2012 Indian Ocean earthquake sequences. In conventional Mohr-Coulomb failure theory, orthogonal faulting requires a zero frictional coefficient (pressure-insensitive), which is unlikely in the brittle lithosphere. The simulations developed here show that near-orthogonal faults can form in the brittle layer by inheriting the geometry of orthogonal shear zones nucleated in the deep ductile (pressure-insensitive) layer. In particular, if the brittle layer is sufficiently thinner than the ductile fault root, near-orthogonal faulting is preserved at the surface. The preservation is further facilitated by a depth-dependent strength in the brittle layer. Conversely, faults nucleated within the brittle layer are unlikely to form at orthogonal angles. Our model thus offers a possible explanation for orthogonal strike-slip faulting and reveals the significant interactions between the structure of faults in the brittle upper lithosphere and their deep ductile roots.